1. Field of the Invention
The present invention relates to a mechanical filter, more particularly to a mechanical filter comprising a pair of piezoelectric ceramic vibrators which vibrate in a longitudinal mode.
2. Description of the Prior Art
Mechanical filters are widely used as frequency selection devices in which electric signals are transformed into ultrasonic waves and vice versa. This type of filter produces numerous advantages, such as having a high Q compared with electric elements L and C, while maintaining or increasing stability and enabling the device itself to be of a small construction. Because of these advantages, the mechanical filter has been used as a channel filter for channel translating equipment or as a filter for navigation receivers or for PCM data transmission equipment.
Generally, each vibrator, i.e., an electromechanical transducer, having a piezoelectric ceramic material is fabricated in two different ways. First by mounting two piezoelectric ceramic materials separately on the top and bottom sides of a base metal plate, to form a sandwich structure, and second, by fabricating the vibrator from the piezoelectric ceramic material itself. The first type of vibrator is more popular than the second, however, the first type has a shortcoming in that it is not suitable for use as a wide band filter. The present invention specifically refers to a mechanical filter set up in accordance with the second type of vibrator, known as a ceramic filter. This type of ceramic filter is advantageous in cost but suffers from various problems, the first of which is that of poor stability with respect to temperature variations and aging. The second is that metal film electrodes are liable to peel away from the ceramic material because it is impossible to fix a support wire to metal film electrodes without using a soldering technique. Further, care must be taken during the manufacturing process, such as the cutting and grinding of the ceramic material, because the ceramic material itself is of a very brittle nature.
Furthermore, ceramic filters, and other mechanical filters, are usually required to cooperate with an external band pass filter assembled from electric components, i.e., coils and capacitors, in order to fully suppress the deleterious spurious vibration modes, inherent to mechanical filters. In this case, an electric band pass filter is connected at the input or output side of the mechanical filter. Under such circumstances, if such an external filter for suppressing the spurious mode can be eliminated, it would have the great advantage of substantially reducing the size of the filter structure.